ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Summary of Asset Retirement Obligations, Contractual Third-Party Reclamation Receivables, and Reclamation Deposits (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	$ 419,763	$ 452,745	$ 279,864
Contractual Third- Party Reclamation Receivable	94,935
Reclamation Deposits	77,364
Surety bonds outstanding to secure reclamation obligations	384,100
Letters of credit outstanding to secure reclamation obligations	103,200
Coal U.S. Segment [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	251,116
Contractual Third- Party Reclamation Receivable	89,408
Reclamation Deposits	77,364
Coal Canada Segment [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	110,978
Contractual Third- Party Reclamation Receivable	5,527
Reclamation Deposits	0
Coal Westmoreland Resource Partners LP [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	56,634
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	0
Power [Member]
Asset Retirement Obligation [Line Items]
Asset Retirement Obligation	1,035
Contractual Third- Party Reclamation Receivable	0
Reclamation Deposits	$ 0